The present invention relates to linear actuators and, more particularly, to miniature linear actuators.
Linear actuators are well known and widely used to convert torque produced by an electric motor or the like into linear motion. The requirements of certain kinds of uses demand that the linear actuator be miniature in size. Situations requiring miniature linear actuators include space-borne applications where size and weight is crucial, robotics applications and various assembly and instrumentation applications. For these types of applications, a miniature actuator is desired that has high stiffness to resist bending under heavy loads.
In addition, the miniature actuator should be fast acting thereby producing motion without delay and a capability of moving rapidly through its limits of motion. Finally, the miniature actuator should be well suited for commercially available components to be used therewith such as miniature electric motors which are high rotation rate and low torque devices, and gearboxes that have fixed reduction ratios.
Commonly used linear actuators make use of a motor driven screw having a nut thereon. The nut has a sleeve assembly attached thereto. The sleeve extends from a housing which prevents the sleeve from rotating with the nut while allowing the sleeve to be extended from and returned toward the housing. Rotation of the screw produces forces on the nut acting parallel to the longitudinal axis of the screw to force the nut to move linearly along the screw and thereby extend or retract the sleeve.
An example of this type of linear actuator is found in U.S. Pat. No. 4,392,390 to Johnson.While this mechanism employs one method of converting torque to linear motion, it also serves to identify several drawbacks the screw and nut system has when used for miniature applications. Bending forces resulting from load forces acting on the extended sleeve must be borne by the screw shaft. The stiffness of the screw shaft decreases with a decrease in diameter of the shaft. Therefore, such miniaturized screw and nut type linear actuators have reduced stiffness, i.e. resistance to bending. In addition, a miniaturized screw and nut type actuator requires the use of small shaft bearings that are not capable of withstanding either high bending forces acting in the direction perpendicular to the shaft axis or forces acting along the screw axis.